Code division multiple access mobile communication system

ABSTRACT

In a mobile communication system which uses a code division multiple access (CDMA) scheme for communications between a base station and a plurality of mobile stations, the base station has a transmitting device in which a plurality of information sequences S1 through Sn are respectively spread by multipliers 11 through 1n with a common spreading code from a spreading code generator circuit 10, the spread codes are provided to transmitting timing control circuits 21 through 2n and then added by an adder 30 to perform transmitting timing offset multiplexing and then the spread signals are transmitted to the mobile stations at different transmitting timing. The mobile stations each have a receiving device which receives that one of the transmitted signals which was transmitted at timing predetermined for the mobile station and despreads the received signal with the same spreading code as that used in the transmitting device, thereby reconstructing the original information sequence concerned.

TECHNICAL FIELD

The present invention relates to a mobile communication system whichutilizes a code division multiple access (CDMA) system forcommunications between a base station and a plurality of mobilestations.

PRIOR ART

The code division multiple access (CDMA) system is a communicationsystem which multiplexes information sequences to be transmitted at thesame frequency, by spreading them with different spreading codes forrespective channels; for example, in literature 1: R. C. Dixon, "SpreadSpectrum Communication System," published by Jatec, there are describedin detail its system configuration and capabilities. Now, a briefdescription will be given of a system called a direct sequence CDMAsystem.

FIG. 19 shows the construction of a transmitting device in a typicalCDMA communication system. n information sequences S1, S2, . . . , Snare input into multipliers 11, 12, . . . 1n, wherein they are spread byspreading codes C1, C2, . . . , Cn from a spreading code generatorcircuit 2, respectively. The output signals from the multipliers 11, 12,. . . , 1n are added by an adder 3 at the same timing to generate atransmission signal.

On the other hand, the signal received at the receiving side is despreadby the same spreading codes C1, C2, . . . , Cn as those used in thetransmitting device, by which the original information sequences S1, S2,. . . , Sn are generated. As the spreading codes C1, C2, . . . , Cn,code sequences are usually chosen which are excellent not only inself-correlation characteristic (letting the time offset between twocorrelating code sequence be represented by τ, the correlation value islarge for τ=0 and small for other values of τ) but also inmutual-correlation characteristic, that is, low in the correlation withother spreading codes (code sequences whose mutual-correlation value issmaller than a certain value which is offset for any given time).

On the other hand, there has been proposed a scheme of repeatedly usingthe same spreading code in a plurality of cells in an application of theCDMA system to a multi-cellular mobile communication system (JapanesePat. Pub. No. 56290/83). It is expected that this scheme will make itpossible to increase the number of communication channels and the systemcapacity.

However, very few classes of code sequences satisfy the above-mentionedrequirements for spreading codes and the number of spreading codes ineach class is also small. Accordingly, the conventional CDMAcommunication system which assigns a different spreading code to eachcommunication channel is inevitably limited in the number of channelsavailable for communication, and hence is not suitable for use as aradio telecommunication system of large channel capacity such as amobile radio communication system.

With the scheme of reusing the same spreading code in a plurality ofcells in the multi-cellular mobile communication system, the reuse ofthe same spreading code in adjacent cells that are not sufficiently farapart would degrade the channel or speech quality owing to interferenceor interactions, thus imposing limitations on the increase in thechannel capacity.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a CDMA mobilecommunication system which makes it possible to effectively increase thenumber of communication channels through use of a limited number ofspreading codes.

The mobile communication system according to the present invention whichutilizes the code division multiple access scheme for communicationsbetween at least one base station and a plurality of mobile stations,characterized in:

that the base station has a transmitting device which spreads aplurality of information sequences by the same spreading code and thentransmits them to the plurality of mobile stations at different timing;

that the mobile stations each have a receiving device which receivesthat signal from the transmitting devices of the base station which wastransmitted thereto at timing predetermined therefor and despreads thereceived signal by the same spreading code as that used in thetransmitting device, thereby reconstructing the original informationsequence;

that the base station has a plurality of transmitting devices and usesdifferent spreading codes therefor; and

that such base stations are installed at a plurality of places and thoseat least two predetermined ones of these base stations which arespatially far apart are allowed to use the same spreading code.

The CDMA mobile communication system according to the present inventionis also characterized in:

that a plurality of base stations each have a transmitting device whichspreads an information sequence by a common spreading code and transmitsthe spread signal to mobile stations at timing different for each basestation;

that the mobile stations each have a receiving device which receivesthat signal from the transmitting devices of the base stations which wastransmitted at timing predetermined therefor and despreads the receivedsignal by the same spreading code as that used in the transmittingdevice to obtain the original information sequence;

that the transmitting device of each base station spreads a plurality ofinformation sequences by a plurality of spreading codes and transmitsthe spread signals to the mobile stations at the same timing; and

that those at least two predetermined ones of these base stations whichare spatially far apart are allowed to transmit the spread signals atthe same timing.

The CDMA mobile communication system according to the present inventionis further characterized in:

that a base station has a transmitting device which spreads a pluralityof information sequences by the same spreading code and transmits thespread signals to mobile stations at timing offset by a transmittingtiming offset value preset in accordance with the transmitting powerused or the size of a cell which is the coverage of the base station;and

that mobile stations each have a receiving device which receives thatsignal from the transmitting device of the base station which wastransmitted at timing predetermined therefor and despreads the receivedsignal by the same spreading code as that used in the transmittingdevice to obtain the original information sequence.

In a first embodiment of the CDMA mobile communication system accordingto the present invention, a plurality of information sequences arespread by the same spreading code in the base station and the spreadsignals are transmitted therefrom to a plurality of mobile stations atdifferent timing; that is, the transmitting timing offset is multiplexedto provide each spread signal with a transmitting timing offset peculiarthereto. By this, communication channels of the same number as that ofthe transmitting offsets are formed although the same spreading code isused.

On the other hand, each mobile station receives that transmitted signalwhich was transmitted at timing predetermined for the mobile station anddespreads the received signal by the same spreading code as that used inthe transmitting device, by which the original information sequence iseasily reconstructed.

Moreover, in the case of using different spreading codes in a pluralityof transmitting devices provided in one base station, it is possible tocommunicate with mobile stations at the same timing by simultaneouslyusing the different spreading codes--this provides an increased numberof communication channels.

Furthermore, in the case of using the same spreading code in thetransmitting devices of two base stations spatially far apart, theso-called spatial re-use of spreading codes and the transmitting timingoffset multiplexing together provide increased system capacity.

In a second embodiment of the CDMA mobile communication system accordingto the present invention, information sequences are spread by a commonspreading code in a plurality of base stations and are transmitted to aplurality of mobile stations at timing different for each base station.In this case, a basic symbol timing interval on which the transmittingtiming for the base stations is based is selected such that the use ofthe same spreading code in a plurality of base stations will not causeinterference. Letting the number of base stations using the commonspreading code be represented by N, there will be established Nindependent communication channels for the same spreading code.

On the other hand, each mobile station receives the signal which wastransmitted at timing predetermined for the mobile station and despreadsthe received signal by the same spreading code as that used in thetransmitting device, by which the original information sequence iseasily reconstructed.

Moreover, in the case of using a plurality of different spreading codesin the transmitting device of the base station, it is possible totransmit a plurality of information sequences by simultaneously usingthese different spreading codes--this provides an increased number ofcommunication channels.

Furthermore, by the spatial re-use of transmitting timing thattransmitting devices of two base stations spatially far apart areallowed to transmit spread signals at the same timing, it is possible touse more communication channels throughout the entire system--thisprovides increased system capacity.

In further embodiment of the CDMA mobile communication system accordingto the present invention, a plurality of information sequences arespread by the same spreading code in a base station and the spreadsignals are transmitted to a plurality of mobile stations at differenttiming thereby multiplexing the transmitting timing offset to provideeach spread signal with a transmitting offset peculiar thereto. In thisinstance, the transmitting timing offset interval is selected inaccordance with the transmitting power or cell size of the base station;for example, in a base station whose transmitting power or cell issmall, the transmitting timing offset interval is set short to permitaccommodation of many mobile stations, whereas in a base station whosetransmitting power or cell is large the transmitting timing offsetinterval is set long to avoid overlapping of delay profiles. Thus, thechannel quality can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the configuration of atransmitting device provided in a base station in a first embodiment ofthe present invention.

FIG. 2 is a diagram showing an example of a transmitting timing offsetin the first embodiment.

FIG. 3 is a block diagram illustrating the configuration of a receivingdevice provided in a mobile station in the first embodiment.

FIG. 4 is a graph showing an example of a correlation output from amatched filter in FIG. 3.

FIG. 5 is a diagram showing, by way of example, a combination oftransmitting timing and a plurality of spreading codes in the case ofusing the spreading codes in one base station in a second embodiment ofthe present invention.

FIG. 6 is a diagram showing, by way of example, a plurality of basestations, a plurality of cells formed by them and a combination oftransmitting timing and spreading codes in each base station inaccordance with a third embodiment of the present invention.

FIG. 7 is a block diagram illustrating the configuration of atransmitting device provided in a base station in a fourth embodiment ofthe present invention.

FIG. 8 is a diagram showing an example of setting a basic symbol timingin the fourth embodiment.

FIG. 9 is a diagram showing an example of spreading codes for use inrespective base stations in the fourth embodiment.

FIG. 10 is a block diagram illustrating the configuration of a receivingdevice provided in a mobile station in the fourth embodiment.

FIG. 11 is a graph showing an example of the correlation output from amatched filter in FIG. 10.

FIG. 12 is a diagram for explaining the re-use of basic symbol timing ina fifth embodiment of the present invention.

FIG. 13 is a block diagram illustrating the configuration of atransmitting device provided in a base station in a sixth embodiment ofthe present invention.

FIG. 14 is a diagram showing two cells of different sizes in a mobilecommunication system.

FIG. 15 is a diagram showing an example of a transmitting timing offsetand the correlation output when delay profiles at the receiving sideoverlap.

FIG. 16 is a graph showing, by way of example, transmitting timingoffset in base stations of cells 1 and 2 in the sixth embodiment.

FIG. 17 is a block diagram illustrating the configuration of a receivingdevice provided in a mobile station in the sixth embodiment.

FIG. 18 is a graph showing, by way of example, the correlation output inbase stations of cells 1 and 2 in the sixth embodiment.

FIG. 19 is a block diagram illustrating the configuration of atransmitting device in a conventional CDMA communication system.

DISCLOSURE OF THE INVENTION (Embodiment 1)

FIG. 1 illustrates in block form the construction of a transmittingdevice which is provided in a base station in Embodiment 1, fortransmitting to n mobile stations at the same time.

In FIG. 1, a spreading code generator circuit 10 is controlled by acontroller 20 to generate a spreading code in synchronism with each ofinformation sequences S1, S2, . . . , Sn. Multipliers 11, 12, . . . , 1nmultiply the information sequences S1, S2, . . . , Sn with the samespreading code supplied from the spreading code generator circuit 10 tospread the information sequences S1, S2, . . . , Sn. Transmitting timingcontrol circuits 21, 22, . . . , 2n are controlled by the controller 20to delay (or offset) the output signals from the multipliers 11, 12, . .. , 1n in correspondence to the n mobile stations, respectively, therebycontrolling the transmitting timing. This operation is called atransmitting timing offset. An adder 30 adds the output signals from thetransmitting timing control circuits 21, 22, . . . , 2n into atransmission signal.

Next, the operation of this transmitting device will be described. Theinformation sequences S1, S2, . . . , Sn, which are to be transmitted,are spread by the multipliers 11, 12, . . . , 1n with the same spreadingcode from the spreading code generator circuit 10 and delayed by thetransmitting timing control circuits 21, 22, . . . , 2n for transmittingtiming control, thereafter being added by the adder 30 into atransmitting timing offset multiplexed transmission signal. Thetransmission signal thus generated is output from a transmission circuitpart, not shown, and is then transmitted via a radio circuit to the nmobile stations.

FIG. 2 shows an example of the above-mentioned transmitting timingoffset. In a period of time defined by one symbol length Ts of theinformation sequences S1, S2, . . . , Sn a plurality (n) of basic symboltiming T1 through Tn are set and delays corresponding to the respectivebasic symbol timing are provided in the transmitting timing controlcircuits 21, 22, . . . , 2n. Letting A represent a basic time, the delayis d1 at the timing T1, d2 at T2 and dn at Tn. The interval of thistransmitting timing is set to a value substantially corresponding to thesum of a delay spread and a guard time at the time of reception by themobile station.

FIG. 3 illustrates in block form the construction of a receiving devicewhich is provided in each of the n mobile stations to receive the signalwhich is transmitted from the transmitting device shown in FIG. 1. Thereceiving device of this example employs a matched filter as acorrelator. The illustrated receiving device comprises a matched filter31, a demodulator 32, a timing extracting circuit 33 and a controller34. The matched filter 31 detects the correlation between a spreadingcode from the controller 34 and the received signal. The timingextracting circuit 33 extracts the received timing of the receivedsignal from the correlation output of the matched filter 31 andindicates the received timing to the demodulator 32.

The demodulator 32 reconstructs the original information sequence Si(i=1, 2, . . . , n) from the output of the matched filter 31 and theoutput of the timing extracting circuit 33. A typical demodulator is aRAKE demodulator, which is described in detail in, for example,literature 2: U. Grob, A. L. Welti, E. Zollinger, R. Kung and H.Kaufmann, "Microcellular Direct-Sequence Spread-Spectrum Radio SystemUsing N-Path RAKE Receiver," IEEE JSAC, Vol. SAC-8, No. 5, pp. 772-780,June, 1990.

The controller 34 supplies the spreading code to the matched filter 31and provides local information at the time of extracting the receivedtiming by the timing extracting circuit 33.

Next, the operation of the receiving device will be described. Thereceived signal, that is, the signal transmitted from the transmittingdevice in the base station of the FIG. 1 construction and received bythe receiving device of FIG. 3, is applied first to the matched filter31. The matched filter 31 is being supplied with the same spreading codeas that generated by the spreading code generator circuit 10 in thetransmitting device of FIG. 1 and correlates the spreading code with thereceived signal to despread the latter, generating the correlatedoutput. FIG. 4 shows an example of the correlated output from thematched filter 31, the abscissa representing time and the ordinate thecorrelated value. Since the signal received by the receiving device ofFIG. 3, that is, the transmitted signal from the transmitting device ofFIG. 1, has been subjected to the transmitting timing offsetmultiplexing as described previously, large correlated values aredetected at the points in time corresponding to the basic symbol timingT1 through Tn.

The timing extracting circuit 33 is presupplied, from the controller 34,with information about the timing for receiving the signal that istransmitted to this mobile station, for example, information about thereference time A and the time length therefrom to the basic symboltiming Ti (i=1, 2, . . . , n); the timing extracting circuit uses thisinformation to recognize the receiving timing in this station from theoutput of the matched filter 31. Upon detecting that this receivingtiming is reached, the timing extracting circuit 33 indicates it to thedemodulator 32. The demodulator 32 reconstructs the information sequenceSi transmitted to this station, from the output of the matched filter 31at the receiving timing indicated by the timing extracting circuit 33.

In a direct spread-code division multiple access (DS-CDMA) system usingspreading codes, even if signals are simultaneously transmitted to aplurality of mobile stations which use the same spreading code, eachmobile station is capable of capturing only the portion of a profile tobe received and reconstructing the information sequence unless delayprofiles of the transmitted signals overlap. This means that theinformation sequence in each channel can independently be demodulatedand decoded regardless of the transmitting timing offset multiplexing.

Thus, the transmitting timing offset multiplexing as in this embodimentprovides independent communication channels of the same number as thetransmitting timing offset number n although only one spreading code isused in common to a plurality of mobile stations. Hence, it is possibleto obtain communication channels n times as many as in the past by useof a limited number of spreading codes of excellent self-correlation andmutual correlation characteristics--this permits implementation of aCDMA mobile communication system of large channel capacity.

Incidentally, the demodulator 32 may also be an ARD demodulator such asdescribed in literature 3: Akihiro Higashi, Tadashi Matsumoto, "BERPerformance of Adaptive RAKE Diversity (ARD) in DPSK DS/CDMA MobileRadio," The Transactions of the Institute of Electronics, Informationand Communication Engineers of Japan, SST92-16, June, 1992 or literature4: Japanese Pat. Appln. No. 83947/92 "Spread Spectrum Receiver." Withthe use of the ARD demodulator, even if the delay profiles of thetransmitted signals overlap to some extent, it is possible toreconstruct the transmitted signal sequence--this enables thetransmitting timing offset to be effected at reduced intervals and henceincreases the transmitting timing offset number n accordingly.

While this embodiment uses a matched filter as the correlator, acombined version of sliding correlators can also be used as long as itis able to detect a plurality of basic symbol timing.

(Embodiment 2)

Embodiment 1 has been described in connection with the case where onebase station uses one spreading code, and consequently, Embodiment 1provides communication channels of the same number as the transmittingtiming offset number n as described above.

In contrast thereto, according to Embodiment 2, each base station has aplurality of transmitting devices of the FIG. 1 construction, which arecommon in the use of the plurality of transmitting timing (T1-Tn) butuse different spreading codes. The respective spreading codes are codesequences each of which has excellent self-correlation andmutual-correlation characteristics as referred to previously. In thisinstance, letting the number of transmitting devices provided in onebase 10 station be represented by m, the number of availablecommunication channels is m×n.

FIG. 5 shows, by way of example, such transmitting timing and spreadingcodes. In FIG. 5, the base station is shown to use n transmitting timingT1 through Tn and four spreading codes C1 through C4; accordingly, thenumber of available communication channels is 4×n.

(Embodiment 3)

While Embodiment 1 has been described in respect of the CDMA mobilecommunication system wherein each base station has one transmittingdevice and adopts the transmitting timing offset scheme, Embodiment 3will be described in connection with the CDMA mobile communicationsystem which employs a plurality of base stations.

FIG. 6 shows an example of a multi-cellular mobile communication system,together with examples of transmitting timing and spreading codes foruse in each base station. T11 through T1n are transmitting timing in abase station 1 and Ti1 through Tin transmitting timing in a basestation 1. When spreading codes of excellent self-correlation andmutual-correlation characteristics, such as referred to previously, areused, substantially no interference or interaction occurs betweenchannels of different spreading codes; hence, there is no need of usingdifferent timing for each base station i. That is, partial or fullcoincidence of respective transmitting timing T11-T1n, T21-T2n, . . . ,TJ1-TJn would not matter.

Furthermore, as described in literature 5: "Fundamentals of MobileCommunication," edited by the Institute of Electronics, Information andCommunication Engineers of Japan, electric waves attenuate in inverseproportion to the square of the distance in the free space and about thefourth power of the distance in a city area, and therefore, nointerference occurs between places spatially far apart to some extent,even if the same spreading code is used. This can be utilized forspatial re-use of spreading codes. For example, as shown in FIG. 6, thebase station J sufficiently far apart from the base station 1 is allowedto use the same spreading code as that used by the latter.

With the combined use of such spatial re-use of spreading codes and theaforementioned transmitting timing offset multiplexing, it is possibleto increase the number of spreading codes available throughout theentire system. Hence, the combined use of such two schemes provides anincreased number of communication channels available for use and permitsimplementation of a CDMA mobile communication system of very largechannel capacity.

Incidentally, it is also possible to combine the system configurationsof Embodiments 1 and 2.

(Embodiment 4)

FIG. 7 illustrates in block form the construction of a transmittingdevice which is provided in each base station in this embodiment. Thetransmitting device is shown to transmit to n mobile stations at thesame time.

The transmitting device comprises a spreading code generator circuit110, multipliers 111, 112, . . . , 11n, an adder 121, a transmittingtiming control circuit 122, a controller 123 and a synchronizationtiming generator circuit 124. The spreading code generator circuit 110generates spreading codes C1, C2, . . . , Cn in synchronization with theinformation sequences S1, S2, . . . , Sn, respectively. The multipliers111, 112, . . . , 11n multiply the information sequences S1, S2, . . . ,Sn by the spreading codes C1, C2, . . . , Cn from the spreading codegenerator circuit 110, respectively, thereby spreading the informationsequences S1, S2, . . . , Sn. The adder 121 adds together the outputsignals from the multipliers 111, 112, . . . , 11n. Based onsynchronization timing (basic timing) from the synchronization timinggenerator 124, the transmitting timing control circuits 122 controls thetransmitting timing to transmit the output signal of the adder 121 atbasic symbol timing instructed by the controller 123.

Next, the operation of this transmitting device will be described. Theinformation sequences S1, S2, . . . , Sn to be transmitted are addedtogether by the adder 121 after being spread by the multipliers 111,112, . . . , 11n with the spreading codes C1, C2, . . . , Cn from thespreading code generator circuit 110. The output signal of the adder 121is controlled in transmitting timing by the transmitting timing controlcircuit 122 to provide a transmission signal. The thus producedtransmission signal is output from a transmitting circuit part, notshown, and is then transmitted via radio channels to n mobile stations.The transmitting timing is adjusted so that the basic symbol timing doesnot coincide among a plurality of base stations.

FIG. 8 shows an example of the setting of the basic symbol timing ineach base station. As shown in FIG. 8, a plurality of basic symboltiming points T1, T2, . . . , Tn are set in the period of one symbollength Ts of the information sequence, and a delay from the basic timingA of each basic symbol timing is provided by the transmitting timingcontrol circuit 122 for each base station. That is, in the case wherethe base station 1 transmits at the timing T1, the base station 2 at thetiming T2 and the base station Tn at the timing Tn, the timing T1 isdelayed by d1 relative to the basic timing A, T2 by d2 and Tn by dn. Theinterval of these basic symbol timing points T1, T2, . . . , Tn is setto a value substantially corresponding to the sum of the delay spreadand guard time at the time of receiving a signal by each mobile station.

In this embodiment, as shown in FIG. 9, the base stations 1, 2, . . . ,n are each allowed to use the common spreading codes C1, C2, . . . , Cn,making it possible to substantially increase communication channelsavailable for use. Moreover, to allow respective base stations to usecommon spreading codes provides a significant advantage in terms ofsystem construction and operation that management of the spreading codesis simple and easy or unnecessary.

FIG. 10 illustrates in block form the construction of a receiving devicewhich is provided in each of n mobile stations for receiving the signaltransmitted from the transmitting device of FIG. 7. The receiving devicein this example uses a matched filter as the correlator. This receivingdevice comprises a matched filter 131, a demodulator 132, a basic symboltiming extracting circuit 133 and a controller 134. The basic symboltiming extracting circuit 133 extracts from the correlated output of thematched filter 131 the basic symbol timing of the signal transmitted tothis station and indicates the extracted timing to the demodulator 132.

The demodulator 132 uses the outputs from the matched filter 131 and thebasic symbol timing extracting circuit 133 to reconstruct the originalinformation sequence Si (i=1, 2, . . . , n). The demodulator may be theRAKE demodulator described in the aforementioned literature 2.

The controller 134 provides spreading codes to the matched filter 131and local information to the basic symbol timing extracting circuit 133when extracting the basic symbol timing.

Next, a description will be given of the operation of this receivingdevice which is provided in each mobile station. The receiving signal,that is, the signal transmitted from the transmitting device of the FIG.7 construction in the base station and received by the receiving deviceof FIG. 10, is fed first to the matched filter 131. The matched filter131, which is being supplied with the same spreading code as thatgenerated by the spreading code generator circuit 110 in thetransmitting device of FIG. 7, correlates the spreading code with thereceived signal to despread the latter, generating a correlated output.

FIG. 11 shows an example of the correlated output from the matchedfilter 131, the abscissa representing time and the ordinate thecorrelation value. Since the signal received by the receiving device ofFIG. 10, that is, the transmission signal from the transmitting deviceof FIG. 7, was transmitted at offset basic symbol timing points T1through Tn for each base station as referred to previously, largecorrelation values are detected at points in time corresponding to thebasic symbol timing points T1 through Tn.

The basic symbol timing extracting circuit 133 is presupplied, from thecontroller 134, with information about the basic symbol timing of thesignal transmitted to this station, for example, information about thebasic symbol timing A and the time length therefrom to the basic symboltiming Ti (i=1, 2, . . . , n); the basic symbol timing extractingcircuit uses this information to recognize from the output of thematched filter 131 the basic symbol timing at which the transmittedsignal is to be received by this station. Upon detecting that the basicsymbol timing is reached, the basic symbol extracting circuit 133indicates it to the demodulator 132. The demodulator 132 reconstructsthe information sequence Si transmitted to this station, from the outputof the matched filter 131 at the basic symbol timing indicated from thebasic symbol timing extracting circuit 133.

In a direct spread-code division multiple access (DS-CDMA) system usingspreading codes, even if signals are simultaneously transmitted to aplurality of mobile stations which use the same spreading code sequence,it is possible for each mobile station to reconstruct the originalinformation sequence by capturing that portion of a profile which is tobe received by the mobile station, unless the delay profiles of thetransmitted signals overlap. This means that even if adjacent cells oradjoining base stations use the same spreading code in themulti-cellular mobile communication system, the information sequences inrespective channels can independently be demodulated or decoded unlessthe basic symbol timing points overlap.

Hence, by setting the basic symbol timing different for each of the nbase stations as in this embodiment, it is possible to use one spreadingcode in common to all the base stations and obtain n independentcommunication channels. By this, it is possible to obtain communicationchannels of a number n times larger than in the prior art by use of alimited number of spreading codes of excellent self-correlation andmutual-correlation characteristics, permitting the implementation of aCDMA mobile communication system of large channel capacity.

Incidentally, the demodulator 132 may also be formed by, for instance,the ARD demodulator described in literature 3 or 4. The use of this ARDdemodulator permits reconstruction of the transmitted signal sequence,even if the delay profiles overlap to some extent. Accordingly, thenumber n of communication channels can be increased by furtherdecreasing the interval between the basic symbol timing points of therespective base stations.

While this embodiment uses a matched filter as the correlator, acombined version of sliding correlators can also be used as long as itis able to detect a plurality of basic symbol timing points.

(Embodiment 5)

FIG. 12 shows an example of a multi-cellular mobile communicationsystem, together with examples of basic symbol timing and spreadingcodes for use in each base station. T1 is the basic symbol timing in abase station 1 and Ti is similarly the basic symbol timing in a basestation i (i=1, 2, . . . ). By setting the basic symbol timing differentfor each base station, no interference occurs between the base stationsas mentioned previously with respect to Embodiment 3, and consequently,the same spreading codes C1-Cn can be used in common to different basestations.

Furthermore, as described in literature 5, since electric wavesattenuate in inverse proportion to the square of distance in the freespace and about the fourth power of distance in a city area, nointerference occurs between places spatially far apart to some extent,even if the same basic symbol timing is used. This can be utilized tospatially re-use the same basic symbol timing. For example, as shown inFIG. 12, a base station J sufficiently far apart from the base station 1is allowed to use the same basic symbol timing as that in the basestation 1.

With such spatial re-use of the basic symbol timing, it is possible toprovide an increased number of communication channels available for usethroughout the mobile communication system. Accordingly, the combineduse of the spatial re-use of the basic symbol timing and the offset ofthe basic symbol timing described previously with respect to Embodiment4 will further increase the number of communication channels availablefor use, permitting implementation of a CDMA mobile communication systemof very large channel capacity.

(Embodiment 6)

FIG. 13 illustrates in block form the construction of the transmittingdevice which is provided in the base station in this embodiment. Thetransmitting device is shown to transmit to n mobile stations at thesame time.

In FIG. 13, a spreading code generator 210 is controlled by a controller231 to generate a spreading code in synchronization with each of theinformation sequences S1, S2, . . . , Sn. Multipliers 211, 212, . . . ,21n multiply the information sequences S1, S2, . . . , Sn by the samespreading code from the spreading code generator 210 to spread theinformation sequences S1, S2, . . . , Sn.

Transmitting timing control circuits 221, 222, . . . , 22n arecontrolled by the controller 231 to delay (or offset) the output signalsof the multipliers 211, 212, . . . , 21n by different valuescorresponding to the n mobile stations, respectively, therebycontrolling the transmitting timing. This operation is called atransmitting timing offset. More specifically, the controller 231instructs the basic symbol timing interval to the transmitting timingcontrol circuits 221, 222, . . . , 22n. The transmitting timing controlcircuits 221, 222, . . . , 22n each set the instructed basic symboltiming interval and indicate it to the controller 231.

An adder 233 adds output signals of the transmitting timing controlcircuits 221, 222, . . . , 22n into a transmission signal, which isprovided to a transmitting circuit 234. The transmitting circuit 234receives a transmitting power specifying instruction from the controller231 and transmits with the specified transmitting power. The controller231 supplies the transmitting timing control circuits 221, 222, . . . ,22n with an instruction specifying the aforementioned basic symboltiming interval in accordance with the transmitting power specifyinginformation that is sent to the transmitting circuit 234. That is, thecontroller 231 sets the basic symbol timing interval wide or narrow,depending on whether the specified transmitting power is large or small.

FIG. 14 shows two cells to which two base stations belong, respectively.Assume that the transmitting power in cell 1 is larger than in cell 2. Atransmitting timing setting part 232 sets therein the basic symboltiming interval which is indicated to the controller 231 in accordancewith the transmitting power as described above. By this, thetransmitting offset interval is set in accordance with the transmittingpower. In this embodiment, the base stations are assumed to besubstantially identical in conditions other than the transmitting power,such as the landform of their sites, the height and directivity of theirantennas and so forth. In this instance, it is expected that the servicearea of the cell 1 is wider than the service area of the cell 2 as shownin FIG. 14.

Next, the operation of this transmitting device will be described. Theinformation sequences S1, S2, . . . , Sn to be transmitted are spread bythe multipliers 211, 212, . . . , 21n with the same spreading code fromthe spreading code generator circuit 210, then delayed by thetransmitting timing control circuits 221, 222, . . . , 22n foradjustment of their transmitting timing and combined by the adder 233into a transmission signal multiplexed at the transmitting timing offsetintervals corresponding to the transmitting power. The transmissionsignal thus produced is output from the transmitting circuit 234 andtransmitted via radio channels to n mobile stations.

In the mobile communication system, as the cell becomes larger, thepossibility of the base station and the mobile station getting far apartincreases and the mobile station is likely to receive a multipath of alarge delay time. In this case, if the transmitting device of the basestation performs the transmitting timing offset by the basic symboltiming offset as shown in FIG. 15(a), a delay spread abruptly increasesin the mobile station having received the multipath of a large delay.Consequently, when the basic symbol timing offset interval is small, thecorrelated output in the receiving device of the mobile station maysometimes become such as shown in FIG. 15(b) in which delay spreads ofcorrelated outputs at adjacent basic symbol timing points overlap,making it impossible to reconstruct the original information sequencecorrectly.

The present invention avoids such a problem by setting the transmittingtiming offset interval in accordance with the transmitting power. FIGS.16(a) and (b) show, by the basic symbol timing, examples of transmittingtiming offset in the transmitting device of the base stations belongingto the cells 1 and 2 in this embodiment.

In the transmitting device of the base station belonging to the cell 1,as depicted in FIG. 16(a), a plurality (n) of basic symbol timing pointsT11-T1n are set in the period of time defined by one symbol length Ts ofthe information sequences S1, S2, . . . , Sn and the delayscorresponding to the respective basic symbol timing points are providedby the transmitting control circuits 221, 222, . . . , 22n. Letting Arepresent the basic or reference time, the delays at the respectivebasic symbol timing points relative to the reference time A are d1 atthe timing T11, d12 at T12 and d1n at T1n. The time interval between thebasic symbol timing points T11, T12, . . . , T1n is set to a valuesubstantially corresponding to the sum of the display spread and theguard time at the receiving time of the mobile station in the servicearea estimated from the magnitude of the transmitting power of thetransmitting device of the base station.

Also in the transmitting device of the base station of the cell 2, asshown in FIG. 16(b), a plurality (n) of basic symbol timing pointsT21-T2n are similarly set in the period of time defined by one symbollength Ts of the information sequences S1, S2, . . . , Sn and the delayscorresponding to the respective basic symbol timing points are providedby the transmitting timing control circuits 221, 222, . . . , 22n. Thedelays at the respective basic symbol timing points relative to thereference time A are d21 at the timing T21, d22 at T22 and d2n at T2n.The time interval between the basic symbol timing points T21, T22, . . ., T2n is set to a value substantially corresponding to the sum of thedelay spread and the guard time at the receiving time of the mobilestation in the service area estimated from the magnitude of thetransmitting power of the transmitting device of the base station as inthe case of cell 1.

As shown in FIG. 14, the service areas of the cells 1 and 2 differ insize with the transmitting powers of the base stations. In the cell 1 ofthe large service area in which the transmitting power of the basestation is large, taking into account that the delay spread is verylikely to spread over a wide area according to the place in the servicearea, the offset intervals (d11, d12-d11, . . . ) of the basic symboltiming points d11, d12, . . . , d1n are set larger than the offsetintervals (d21, d22-d21, . . . ) of the basic symbol timing points d21,d22, . . . , d2n in the cell 2, as shown in FIGS. 16(a) and (b) .

FIG. 17 illustrates in block form the construction of the receivingdevice which is provided in each mobile station to receive signalstransmitted thereto from the transmitting device of FIG. 13. In thisexample, a matched filter is used as the correlator. The receivingdevice comprises a matched filter 241, a demodulator 242, a basic symboltiming extracting circuit 243 and a controller 244. The matched filter241 detects the correlation between the spreading code supplied from thecontroller 244 and the received signal. The timing extracting circuit243 extracts from the correlated output of the matched filter 241 thereceiving timing of the signal transmitted to this mobile station andindicates the detected receiving timing to the demodulator 242.

The demodulator 242 reconstructs the original information sequence Si(i=1, 2, . . . , n) on the basis of the output of the matched filter 241and the output of the basic symbol timing extracting circuit 243. TheRAKE demodulator is a typical demodulator and is described in detail inthe aforementioned literature 2, for instance.

The controller 244 provides the spreading code to the matched filter 241and local information to the timing extracting circuit 243 at the timeof its timing extraction. A receiving timing setting part 245 setstherein the receiving timing intervals and indicates them to thecontroller 244.

Next, the operation of this receiving device will be described. Thereceived signal, that is, the signal transmitted from the transmittingdevice of FIG. 13 in the base station and received by the receivingdevice of FIG. 16, is fed first to the matched filter 241. The matchedfilter 241, which is being supplied, from the controller 244, with thesame spreading code as that generated by the spreading code generatorcircuit 210 in the transmitting device of FIG. 13, correlates thespreading code with the received signal to despread the latter,generating a correlated output.

FIGS. 18(a) and (b) show examples of the correlated outputs of thematched filters 241 in the mobile stations belonging to the cells 1 and2, the abscissa representing time and the ordinate the correlated value.As referred to previously, the signal received by the receiving deviceof FIG. 16 has been multiplexed in the transmitting timing offset; sothat large correlated values are detected at the time pointscorresponding to the basic symbol timing points T1 through T1n in thecase of the cell 1 and at the time points corresponding to the basictiming points T21 through T2n in the cell 2.

The basic symbol timing extracting circuit 243 is presupplied, from thecontroller 244, with information about the receiving timing of thesignal transmitted to this mobile station, for instance, informationabout the reference time A and delays at the respective basic symboltiming points relative to the time A; the basic symbol timing extractingcircuit uses this information to recognize from the output of thematched filter 241 the timing at which the transmitted signal is to bereceived by this mobile station. Upon detecting that this receivingtiming is reached, the basic symbol timing extracting circuit 243indicates it to the demodulator 242. The demodulator 242 reconstructsthe information sequence Si transmitted to this station, from the outputof the matched filter 241 at the receiving timing indicated from thebasic symbol timing extracting circuit 243.

As depicted in FIG. 14, the cell 1 is larger in transmitting power thanthe cell 2 and hence has a wider service area, and in the cell 1 thedelay spread is larger than in the cell 2; in this embodiment, thetransmitting timing offset interval in the cell 1 is set larger than inthe cell 2 as shown in FIG. 16. Hence, it is possible, also in the cell1, to reconstruct the information sequence Si by capturing only thatportion of the profile which is to be received from the correlatedoutput of the matched filter 241 in the receiving device of the mobilestation.

In a direct spread-code division multiple access (DS-CDMA) mobilecommunication system using spreading codes, even if signals aresimultaneously transmitted to a plurality of mobile stations which usethe same spreading code sequence, it is possible for each mobile stationto reconstruct the original information sequence by capturing thatportion of a profile which is to be received by the mobile station,unless the delay profiles of the transmitted signals overlap. This meansthat information sequences in the respective channels can independentlybe demodulated or decoded regardless of the transmitting timing offsetmultiplexing.

Hence, independent communication channels of the same number as thetransmitting timing offset number n can be obtained with one spreadingcode by setting the transmitting offset interval (the offset interval ofthe basic symbol timing) in accordance with the transmitting power andperforming the transmitting timing offset multiplexing so that the delayprofiles of the respective information sequences do not overlap. Bythis, it is possible to obtain communication channels of a number ntimes larger than in the past by use of a limited number of spreadingcodes of excellent self-correlation and mutual-correlationcharacteristics, permitting the implementation of a CDMA mobilecommunication system of large channel capacity.

In the above, the transmitting power of the base station is made fixed,but in the case of effecting transmitting power control, the sameresults as described above could be obtained by controlling thetransmitting timing offset interval within the range of the transmittingpower that is controlled in the channel concerned, in accordance withthe maximum transmitting power.

Incidentally, it is also possible to use, as the demodulator, the ARDdemodulator described in literature 3 or 4, for instance.

As described above, the present invention provides the advantages listedbelow.

(1) with the system configuration in which a plurality of informationsequences are spread with a common spreading code, then subjected to thetransmitting timing offset multiplexing and transmitted to a pluralityof mobile stations, it is possible to set up independent communicationchannels of the same number as the transmitting timing offset number.Consequently, even if a code sequence which includes only a limitednumber of spreading codes of excellent self-correlation andmutual-correlation characteristics is used, it is possible to obtaincommunication channels of a number multiplied by the transmitting timingoffset number, permitting the implementation of a CDMA mobilecommunication system of large channel capacity and high flexibility ofchannel selection.

When a plurality of transmitting devices are provided in one basestation and use different spreading codes, communications can be made atthe same transmitting timing points by using the different spreadingcodes at the same time and communication channels of a number (thenumber of transmitting devices by the transmitting timing offset number)can be obtained.

Furthermore, the combined use of the transmitting timing offsetmultiplexing and the spatial re-use of a spreading code enables theimplementation of a CDMA mobile communication system of large channelcapacity and high frequency usage efficiency.

(2) With the system configuration in which information sequences arespread with a common spreading code in a plurality of base stations andtransmitted to mobile stations at timing different for each basestation, even if only one spreading code is used, N independentcommunication channels can be set up when N base stations are providedin the system. Accordingly, even if a code sequence which includes onlya limited number of spreading codes of excellent self-correlation andmutual correlation characteristics is used, it is possible to obtainN-fold communication channels, permitting the implementation of a CDMAmobile communication system of large channel capacity and highflexibility of channel selection.

Moreover, since there is no need of spreading code management such asarranging specific spreading codes for use in a specific base station,the system management load is alleviated; hence, it is possible toconstruct a simple-structured and highly reliable system.

Furthermore, when the transmitting device of the base station uses aplurality of different spreading codes, a plurality of informationsequences can be transmitted at the same timing by using the spreadingcodes at the same time--this provides an increased number ofcommunication channels.

Besides, by the spatial re-use of transmitting timing where thetransmitting device of two base stations spatially far apart use thesame transmitting timing, the entire system is allowed to use anincreased number of communication channels--this permits theimplementation of a CDMA mobile communication system of large channelcapacity and high frequency usage efficiency.

(3) By setting the transmitting timing offset interval in thetransmitting device of the base station in accordance with thetransmitting power or the cell size, that is, by setting thetransmitting timing offset interval small in a base station of smalltransmitting power or cell size and large in a base station of largetransmitting power or cell size, it is possible to avoid overlapping ofdelay profiles and hence ensure high-quality communication.

We claim:
 1. A code division multiple access mobile communication systemwhich uses code division multiple access for communications between aplurality of mobile stations and a base station in each of a pluralityof cells wherein a plurality of different sets each of a plurality ofdifferent spreading codes are each allotted to said cells, respectively,and a same one of sets of said plurality of different spreading codes isallotted to at least two of the plurality of cells spatially far apartfrom each other;the base station in each of said plurality of cellscomprising a plurality of transmitting devices each of which spreads aplurality of information sequences with the same one of said spreadingcodes allotted to the cell to produce a plurality of spread informationsequences, delays the plurality of spread information sequences bycorresponding delay times, respectively, and then transmits saidplurality of delayed spread information sequences, respectively, to saidplurality of mobile stations; and each of said mobile stations in eachcell having a receiving device which receives a signal from said basestation in said cell, despreads the received signal with one of saidspreading codes of one set allotted to the cell to produce a despreadsignal, and extracts information from said despread signal at one ofsaid transmitting timings corresponding to the respective delay times toreproduce an information sequence.
 2. The code division multiple accessmobile communication system according to claim 1, wherein said pluralityof delay times to be used by each of said transmitting devices aredetermined on the basis of the transmitting power of each saidtransmitting device.
 3. The code division multiple access mobilecommunication system according to claim 1, wherein said plurality ofdelay times to be used by each of said transmitting devices of each basestation in each cell are determined on the basis of the size of saidcell constituting the coverage of said base station.
 4. A code divisionmultiple access mobile communication system which uses code divisionmultiple access for communications between a plurality of mobilestations and a base station in each of a plurality of cells wherein aset of different spreading codes is allotted to each of the plurality ofcells and a plurality of predetermined delay times corresponding totransmitting timings are each allotted to the respective cells;said basestation in each of said plurality of cells comprising a transmittingdevice which spreads a plurality of information sequences, respectively,with the different spreading codes of the same set to produce aplurality of spread information sequences, delays the plurality ofspread information sequences by one of the delay times allotted to thecell and then transmits said plurality of the delayed spread informationsequences, respectively, to said plurality of mobile stations; and eachof said mobile stations in each cell having a receiving device whichreceives a signal from said base station in said cell, despreads thereceived signal with one of said different spreading codes of a setallotted to the cell to produce a despread signal, and extractsinformation from said despread signal at one of a plurality of differenttimings corresponding to one of the delay times allotted to the cell toreproduce an information sequence.
 5. The code division multiple accessmobile communication system according to claim 4, wherein saidtransmitting devices of said base stations in at least two of saidcells, which are spatially far apart from each other, use the same delaytime.